1. Field of the invention
The invention concerns a coating product (for example, paint or varnish) installation of the type comprising a pump feeding at least one sprayer. The invention is more particularly concerned with a number of improvements relating to such use of a pump; it is also directed to various applications of the inventive principle in installations of different kinds.
2. Description of the prior art
A known paint spraying installation, preferably provided with a programmed automatic control color changing system, comprises a plurality of circuits conveying paint of different colors, which will be referred to hereinafter as "primary circuits", in each of which paint circulates continuously around a relatively long path, rinsing product feed means, also in the form of a continuous circulation circuit, compressed air supply means and a set of selectively operable valves and control systems forming what is generally called a "color change system" comprising at least one outlet connected to one or more sprayers located in a paint spraying booth. The objects to be painted pass through said booth. This type of installation is more typical of the automobile industry in which one of the most severe operating constraints results from the fact that two successive objects (automobile bodies) in the booth are not generally to be painted the same color, which makes it necessary to be able to carry out an entire color change sequence during a short time period not exceeding the time needed to bring the next object into position in the booth.
In this type of installation it is known to raise the coating products to a relatively high pressure (6 to 7 bars) on the inlet side of the color change system, in other words in the primary circuits, to obtain a sufficiently high pressure at the sprayer. A pressure regulator is generally provided near the sprayer and the conduit in which the paint travels from the color change system to the sprayer is of relatively large cross-section to minimize head losses. This leads to the wastage of non-negligible quantities of paint and rinsing product (solvent) on each color change cycle. The time to perform a color change can also become excessive in certain cases. Moreover, these problems are accentuated if it is necessary to provide a long length of conduit in the booth, which is the case when the sprayer or sprayers is or are mounted on multi-axis robots designed to move inside the booth.
Faced with such difficulties, it has been proposed to insert a pump (and in particular a pump whose flowrate is proportional to the drive speed, of the gear pump type) between the color change system and the paint sprayer or sprayers. As the pump is located near the color change system, head losses in the conduit can be compensated for, irrespective of its length, and it is even possible to reduce significantly the cross-section of the conduit, reducing the wastage of products during a color change cycle, while also reducing the time necessary to perform a color change cycle. It is also possible to reduce the pressure in the primary circuits, typically from 6-7 bars to 3-4 bars. The invention is concerned with improvements to this type of installation. Installations of this kind have been found to be somewhat unreliable, requiring frequent adjustment of the pump flowrate. The invention is the result of investigations into the causes of this lack of reliability.
These showed that the component parts of the gear pump used under such conditions wear more rapidly than in other industrial applications and that this wear results in internal leakage, in other words the existence of a fluid flowrate that is not directly related to the drive speed of the pump. In fact, such leakage exists even when the pump is brand new, although it is then at a very low level; it is its variation in time that eventually compromises correct functioning of the system. The direction and magnitude of such internal leakage depend, among other things, on the pressure difference between the pump inlet and outlet and on the viscosity of the product passing through it. Firstly, the invention provides a solution to this problem.